1. Field of the Invention
The field of the present invention is SOHC type internal combustion engines comprising a pair of intake valves and a pair of exhaust valves, which are carried in a cylinder head, for opening and closing operation, at a small angle formed by axes of the intake and exhaust valves in a projected view on a plane which extends perpendicular to an axis of a crankshaft and includes an axis of a cylinder, a single cam shaft rotatably disposed above a combustion chamber, a single rocker shaft fixedly disposed above the combustion chamber, a plurality of intake rocker arms interposed between the cam shaft and the pair of intake valves and swingably carried on the rocker shaft, a pair of exhaust rocker arms interposed between the rocker shaft and the pair of exhaust valves and swingably carried on the rocker shaft, and a plug insertion cylindrical portion which is provided in the cylinder head either between both the intake valves or between both the exhaust valves and into which a spark plug is inserted to be disposed at a substantially central portion of a ceiling surface of the combustion chamber.
2. Description of the Prior Art
Such SOHC type internal combustion engine has conventionally been known, for example, from Japanese Patent Application Laid-open No. 151104/80.
In the above prior art SOHC type internal combustion engine, the angle formed by the axes of the intake and exhaust valves is set at a small value, thereby defining a flattened combustion chamber to improve a combustion efficiency so as to meet a combustion of a fuel-air mixture having a small concentration of a fuel. However, disposition of the intake and exhaust valves at the small angle results in the fact that the opened area of each of intake and exhaust valve bores must be set at a relatively small level. In addition, the disposition of the plug insertion cylindrical portion either between both the intake valves or between both the exhaust valves results in a further reduced opened area of each of the intake and exhaust valve bores, thereby making it difficult to provide an increased output from the engine.